Last month I contributed a small article for Rice Farming Magazine. I used data generated by the UC Weed Project to illustrate the point of how difficult, complicated and expensive weed control can be once herbicide-resistant weeds are established in a field. Well, a very observant PCA noticed that there was a mistake in my example. Here it is: "However, if dealing with ALS-resistant watergrass and propanil-resistant sedges, this program would be a failure. In this case, a more appropriate program may consist of application of a tankmix of Granite GR and Shark at the 2.5 leaf stage of rice, followed by application of a tankmix of Abolish and Regiment at the 5 leaf stage of rice." Obviously, you can't tankmix Granite GR and Shark! What I meant to say was, a same day application of Granite GR and Shark at the 2.5 leaf stage. This base application gave very good control of umbrellasedge and broadleaves, good control of bulrush, but mediocre control of late watergrass. The follow up application of Abolish and Regiment resulted in excellent control of all weeds.

Apologies for the confusion!

The Weed program at the Rice Experiment Station (RES) in Biggs conducts herbicide resistance testing for the major rice herbicides used in California. This information helps growers improve their weed control programs if resistance is confirmed, and also helps the rice industry as a whole to keep track of resistance issues.

Sprangletop and smallflower umbrella sedge seeds are the first weed seeds to mature. If you suspect herbicide resistance, bring seeds of these weeds to the RES to be tested, together with the Resistant Weed Testing form available here. Follow the guidelines below to ensure that enough seed of the right species is collected. Later in the year we'll update these guidelines to include ricefield bulrush and watergrass.

General guidelines to collect suspected herbicide resistant weed seeds:

• Don't wait until harvest to collect seed. By then, most weeds have shattered their seeds. If you collect after harvest, you'll probably be collecting seeds from weeds growing around the field that may not be the correct species.
• Collect seed when they are mature and dislodge easily from the seedhead. In general, sprangletop matures the earliest, between rice PI and heading. Early watergrass, barnyardgrass, smallflower umbrella sedge, and ricefield bulrush follow, between rice heading and maturity. Late watergrass is the last weed to mature.
• Collect seed from areas of the field where you are certain the herbicide application in question was appropriate. Avoid field borders, tractor tire tracks, skips or areas where you suspect the herbicide was not sprayed correctly.
• Collect seed from several areas of the field. Usually, when herbicide resistance is the problem, weed growth will be distributed uniformly across the field and not just in one “hot spot”.
• Collect seeds, not seedheads. Take the seedhead and gently shake it inside a paper bag. Seeds that shatter are mature and will readily germinate. If seedheads are collected, seeds might not be mature or might have shattered already.

Collect seeds by shaking the seedheads gently into a paper bag

• Collect enough seed. In order to have conclusive results, several replications of the herbicide resistance testing are needed. When not enough seed is provided, replications may not be possible. For small sized seed weed species such as sprangletop, smallflower or sedge, collect seeds from at least 20 mature seedheads. For barnyardgrass, early and late watergrass, collect from at least 30 mature seedheads.
• Monitor seed development. If seeds are not dislodging during collection, they are probably still immature. Return in 2 or 3 days and try again.

Species specific guidelines: 

• Sprangletop seeds shatter easily. Mature seedheads that have shattered their seeds will look dry, while seedheads with immature seeds will look green. Seedheads with mature seeds will have a thicker appearance than empty seedheads.

• There are two sedge species similar to smallflower. Whitemargined flatsedge (Cyperus flavicomus) and ricefield flatsedge (Cyperus iria) grow in field borders and shallow areas in the field. Be careful not to confuse them with smallflower.

Japanese millet is not a weed of rice in our area. However, a PCA recently noticed it in an organic rice field. Japanese millet is in the same genus as our common watergrasses, and looks similar, but is much more robust. It seems to grow well under flooded and dry conditions.

The presence of this plant in the organic rice field was puzzling. Then the PCA learned that the field had been in pasture the year before. Most likely, the Japanese millet was part of the mix of grasses seeded as pasture and produced seed that survived the winter and germinated in the flooded rice field. 

Unusual find, hopefully we won't see it ever again!

Summary

Winged primrose willow is an invasive weed that was identified in Butte County rice fields in 2011. Most infestations were along borders of fields and canals; however, this weed can thrive in the flooded environment within rice fields. Winged primrose willow can disperse through seeds and plant fragments floating in the irrigation water and tillage and harvest equipment.

Greenhouse tests showed that winged primrose willow can be controlled with rice herbicides. Early, into-the-water applications of Bolero Ultramax, Cerano or Granite GR effectively killed young plants. Foliar applications of Sandea and Londax were effective in controlling young plants (1.5 inches). Older plants (2.5 inches) were controlled by Grandstand or a tank mix of Grandstand and SuperWham.

After the initial discovery of winged primrose willow (Ludwigia decurrens) in Butte county, it was determined by Farm Advisors and the Agricultural Commissioner that the infestation expands over several square miles. Most infestations are along borders of fields and irrigation canals. One field had an infestation throughout. It is likely that this weed has gone undetected for up to five years or more. Seed capsules from this plant have thousands of seeds which are capable of floating on the water surface as a means of dispersal, especially along irrigation canals. Indeed, the Butte County Agricultural Commissioner believes this has been the main means of dispersal across the majority of the infested area. Other potential means of spread are by tillage equipment and combines. Additionally, it has been determined that plant fragments have the ability to grow roots within a day or two when in water. This suggests that mowing of levees as a means of control may potentially increase dispersal of this weed.

Testing in the greenhouse at the Rice Experiment Station indicates that the plant germinates best when the soil is moist but not flooded. However, the seed can germinate under water and eventually grow above the water surface with the potential to survive in a rice field and set seed. This plant also has the ability to form roots that grow upwards through the water column in order to scavenge oxygen near the water surface. Glyphosate will control this plant on levees, but any formed seed capsules will have viable seeds that will likely germinate the next season if not removed from the field and buried in a landfill as requested by the Agricultural Commissioner. It is suggested that any levee spraying of known infestations happen early in the season prior to the yellow flowers being visible.

Testing of currently available rice herbicides indicates several potential options for control of this invasive weed within rice fields. We tested early season water active herbicides (Table 1). This was done at early water flood and on larger, more established weeds. The early flood treatment entailed moistened soil for 3 days prior to the flood being established. This was intended to simulate the flooding of large fields where the soil is moist as water is built up for flood. Winged primrose willow will germinate under this conditions prior to flood. Herbicide treatments were applied after the flood was established. The later timing entailed plants that were allowed to establish and grow to approximately 2.5 inches in moist soil, then the flood was established and herbicide treatments applied. This treatment method was intended to be similar to a drill seeded situation where into water herbicides could be applied after establishment of permanent flood.

Table 1. Water active herbicides tested against winged primrose willow at Biggs, CA.

In continuous flood, Cerano caused bleaching and eventual death of the small plants. The later application of Cerano slowly bleached the plants and it is believed that they will not be able to produce viable seed. Bolero Ultramax activity was fairly slow, but eventually killed both young and older plants when flood was maintained. Granite GR slowly bleached the young plants and it is expected to fully control the weed at that stage; however, more established plants survived the treatment and would likely set seed. Shark H₂O initially appeared to be very efficacious on both plant sizes; however, plants were able to survive the treatment by putting on new leaves. These plants eventually flowered and would be expected to produce viable seeds. Sandea applied to the flood water did not control the weed although it caused some malformations of the typical plant. It is likely the plants surviving the Sandea treatment would set seed.

We also tested later season foliar herbicides (Table 2) on both early establishment and larger more established plants. Early establishment in this case consisted of plants allowed to establish to approximately 1.5 inches tall at time of application. The larger, more established plants were approximately 2.5 inches tall.

Table 2. Foliar active herbicides tested against winged primrose willow, Biggs, CA.

Regiment did not control either young or old plants. Granite SC also did not fully control either age of plants. Sandea and Londax, however, appear to control the weed sufficiently when it is young but less effectively when it is more established. Shark H₂O initially appeared to be the best treatment with rapid severe burn of leaves, but the plants recovered by producing new leaves. These plants eventually produced flowers and would be expected to have viable seed. SuperWham provided some control when the plants were small, however the more established plants were not significantly hindered. Grandstand caused severe damage to both young and established plants. The tank mix of SuperWham and Grandstand was the most efficacious of all the foliar treatments.

Conclusions

The best control strategy for winged primrose willow in rice culture would be to use early water active herbicides like Bolero Ultramax, Cerano or Granite GR when the weed is very small and more vulnerable to treatment. If follow-up foliar herbicides are needed, early appications of Sandea or Londax may be sufficient when the weed is still very small. Later foliar applications for control of winged primrose willow would require Grandstand or a tank mix of Grandstand plus SuperWham. Some of the treatments that did not kill winged primrose willow may have been sufficient to prevent establishment in rice and setting of viable seed.